The present invention relates to a laser beam printer, copier, facsimile transceiver or similar image forming equipment and, more particularly, to a device incorporated in such equipment for sequentially separating and feeding cut sheets one by one from a stack.
A predominant type of sheet feed device for the above application uses a pick-up member in the form of a roller or a belt made of a material having a high coefficient of friction, e.g., rubber. The pick-up member feeds sheets on the basis of friction. Another conventional sheet feed device feeds sheets by sucking them, i.e., under vacuum.
The friction type sheet feed device is simple in construction. However, this type of device cannot exert a great frictional force unless a spring or similar resilient member presses the pick-up member against the top of the stack. i.e., uppermost sheet. Another drawback with such a device is that rubber or similar material having a high coefficient of friction is apt to have the coefficient noticeably changed due to the varying ambient conditions, degrading the stability of operation. In addition, since the pressing contact of the pick-up member with the sheet stack is not delicately adjusted, two or more sheets are often fed out together. To eliminate this occurrence, an extra mechanism for surely separating the uppermost sheet from the others is indispensable.
The suction type sheet feed device is advantageous over the friction type device in respect of stable sheet feed. However, the suction scheme generates great noise in the event of suction of air and increases the overall dimensions of the device.
Japanese Utility Model Laid-Open Publication (Kokai) No. 85543/1985 discloses a sheet feed device capable of separating sheets by electrostatic adhesion. Specifically, this sheet feed device has a flat sheet feed member capable of exerting an electrostatic adhering force, a means for moving the flat member substantially in a linear reciprocating motion, and a means for selectively applying or interrupting a voltage for generating or cancelling the adhering force. With this kind of approach, however, it is difficult to separate one sheet from the others due to the leak of electric field, resulting in the simultaneous feed of two or more sheets. In the light of this, Japanese Patent Laid-Open Publication (Kokai) No. 176472/1985 proposes an improved electrode pattern.